1. Field of the Invention
The present invention relates to a Hall sensor for measuring magnetic fields, particularly to a Hall sensor in the form of a monolithic integrated component containing a Hall detector, a power supply, and electronic circuits for evaluating the Hall voltage.
2. Description of the Related Art
To fabricate a Hall sensor comprising a combined detector, power supply and electronics circuit, use is commonly made of one of the conventional silicon semiconductor technologies which correspond to the conventional bipolar or MOS fabrication processes. Known disadvantages of silicon as the Hall detector material are its low Hall sensitivity and the great influence of the piezoelectric effect, which leads to offset-voltage errors caused by stresses in the crystal structure and caused by direction-dependent Hall sensitivity.
The sensitivity of Hall detectors is essentially a function of the carrier mobility and the current flow through the Hall plate. The resolution limit and the spread of measured values are dependent mainly on the variation of the different offset voltages, which depend on the Hall detector itself and the subsequent electronics.
A Hall plate can, to a first approximation, be thought of as a resistance bridge which is adjusted for zero output, i.e., balanced, if no magnetic field is present. Methods for fabricating Hall sensors in which the desired accuracy can only be achieved by a selection from the wide fabrication spectrum are quite unsatisfactory, of course. Therefore, tolerances are kept to a minimum during fabrication by compensation methods. The most widely used method is the compensation of the Hall plate offset voltage by parallel connection of a second Hall plate rotated by 90.degree. which is as equivalent to the first Hall plate as possible. This makes it possible to compensate, to a first approximation, for the imbalance of the resistance bridge due to the piezoelectric effect. Nevertheless, process tolerances or high stress gradients in the crystal prevent a higher precision of the magnetic-field measurement.
A method of compensating the offset voltages is described in "Proceedings of Eurosensors IV", 1991, Vol. 2, pages 747 to 751, which relates to a meeting held in Karlsruhe October 1-3, 1990. The method described therein uses a circular Hall plate with 16 contacts uniformly distributed around the periphery, which are switched circularly by means of electronic switches. The 16 associated Hall voltages are integrated causing the offset error to be reduced to values less than 50 microvolts. In the pertinent description it is stated that it is possible to completely eliminate geometrical errors of the Hall plate by orthogonal switching, but not to eliminate offset errors due to the above-described piezoelectric effects.
Since offset errors in silicon are frequently considerably greater--for instance, up to a factor of 1000--than the Hall voltage to be determined, there is also a specific error of the evaluating circuit.
Assuming an ideal equivalent bridge circuit, during orthogonal switching, the offset error occurs with different signs. If the two orthogonal Hall voltages were summed, the offset error would thus have to be fully compensated. This would apply only if strict proportionality, linearity, equal gain and sensitivity can be assumed for the amplified and processed Hall voltages. The linearity and sensitivity requirements are generally insufficient.